The correct answer is Option A: A is true but R is false.

Let's break down why:

Assertion A: This statement is true. Vertebrates, belonging to the subphylum Vertebrata, do indeed possess a notochord during their embryonic development. This notochord is a flexible rod that provides support. As the embryo develops, the notochord is gradually replaced by a vertebral column, which can be either cartilaginous or bony depending on the species.

Reason R: This statement is false. The statement "Thus all chordates are vertebrates, not all vertebrates are chordates" is incorrect. The relationship between chordates and vertebrates is actually the opposite.

Here's the correct relationship:

• Chordates: This is a phylum that includes all animals possessing a notochord at some point in their life cycle. They also have other key features like a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail.
• Vertebrates: This is a subphylum within the chordates. Vertebrates are distinguished by the presence of a vertebral column.

Therefore, all vertebrates are chordates, but not all chordates are vertebrates. There are other groups within the chordate phylum that lack a vertebral column, such as tunicates and cephalochordates.

In conclusion, Assertion A is a correct statement about the development of vertebrates. However, Reason R misrepresents the relationship between chordates and vertebrates.